This invention relates to techniques for compensating nonlinearities and introducing predetermined variations in the conversion of electronic signals representing video images to images on photographic media.
A number of systems are known for generating color and pseudo-color displays from input video signals presented in raster scan format. Some of these systems process and thereafter convert the video signals derived from image scanning into photographic records of the images more precisely than can direct exposure techniques. A number of factors, some readily evident and others more subtle although still important, preclude obtaining true colors and contrasts when direct exposure is used. It need only be noted that photographic media are inherently nonlinear and that photographic dyes are not only of different colors than CRT phosphors but also function subtractively whereas the color image in a CRT is formed additively. These mismatches at the minimum cause loss of true color, saturation and detail. Conversion of the information contained in video signals to a photographic image not only requires compensation for these and other factors, but also desirably includes a number of other capabilities. It is desirable for example to be able to process images so as to generate negatives as well as positives, pseudo color as well as true color images, and to adjust contrast, hue and luminance to meet individual preferences.
Various workers in the art have resolved these problems with different degrees of success. An outstanding example of a system of general applicability for the conversion of video data to photographic hard copy is provided by the "VIDEOPRINT" system products of Image Resource Corporation of Westlake Village, Calif. As described generally in an article entitled "Microprocessor-Controlled System Prints Color TV Pictures" by Leon Levinson and Edmund Newbert in Electronics magazine for Sept. 22, 1981, pp. 121-125, these systems operate by sequentially generating three different images, one for each color on the photographic medium, on a precision CRT. For each color image a different color filter matched to the corresponding film color is interposed between the CRT and the film. The image is recorded under control of the microprocessor, which determines the exposure time and light intensity variations during each image scan. Because, as described in the article, this system enables many adjustments to be made and includes other features such as raster line elimination, it produces photographic images of high color quality and resolution.
As is typical in modern technology, however, demands are constantly made to achieve further advances in the art with these systems. One significantly desirable feature is the incorporation of a precise but versatile control of light intensity variations with signal amplitude. The need for this function, known generally and hereafter referred to as the gamma function, arises from nonlinearities in the signal transfer paths and the media involved. Because of gamma function nonlinearities, distortions may occur (speaking in monochrome terms) in the white, gray or black contrasts, or in the relationships between them. In color systems, gamma distortion may vary between different colors. The techniques heretofore used for compensating gamma distortion have used nonlinear amplifiers or other compensating circuits in the transfer path, to attempt to achieve an overall linear response. While such techniques can be utilized to optimize a system under one given set of conditions, i.e. a specific device operating with a particular film, this does not satisfy current needs. At most, only coarse "black stretch" or "white stretch" compensation can be introduced in balancing the image. Furthermore, the precision required in these systems means that the minor differences that exist between successive products coming off an assembly line can introduce excessive variations in signal transfer functions. These product-to-product variations can only be compensated by lengthy and expensive individual tuning procedures. Further, such tuning can correct for only one particular set of conditions and other corrections must remain as approximations.
Prior art approaches can therefore be seen to be unsuitable for compensating for the full scope of gamma function nonlinearities arising from cathode ray tube, signal transfer path and photographic media characteristics. They also have limited capability for generating negative/positive images or pseudo-color images and prior expedients also offer only limited versatility with regard to contrast adjustment, color enhancement and other types of color manipulation.